1. Field of the Invention
This invention relates to an image encoding apparatus and a method for encoding image information in a hierarchical form, and more particularly, to an image encoding apparatus and method which are suitable for hierarchical image encoding in, for example, a static-image filing apparatus or a transmission apparatus.
2. Description of the Related Art
A conventional hierarchical image encoding method is described in, for example, "International Standard for Multimedia Encoding" edited by Hiroshi Yasuda (ISBN4-621036149C3055). When transmitting an image using the conventional encoding method, rough image information is transmitted first, followed by transmission of additional information that is used by a receiving device to gradually generate a detailed image.
Conventionally, for performing hierarchical image encoding, an image filing apparatus as shown in FIG. 5 has been used. In such an apparatus, no consideration is given to, for example, the order of generating detailed image information (detailed codes) 114, intermediate image information (intermediate detailed codes) 113, and rough image information (rough codes) 112 shown in FIG. 5.
However, for standard codes, only codes having the structures shown in FIGS. 6, 7, 8, 9, 10 and 11 are permitted. In FIGS. 6 through 11, reference numerals 111, 121, 131, 141 and 151 represent headers, and reference numeral 161 represents an interruption code.
Accordingly, for example, when a code includes only an intermediate detailed code 113 as shown in FIG. 11, the same header is used as in the case when a code also includes a detailed code 114, as shown in FIG. 8. Hence, interruption code 161 is provided in code 160 of FIG. 11 to indicate that code 114 is absent.
Accordingly, codes 130 and 160 cannot be discriminated from each other by examining only header 131. Further, the decoding side cannot determine whether transmitted information comprises code 130 or a combination of codes 160 and 150 until the entire code is transmitted.
In order to prevent such problems, the conventional method preserves information in the form of code 110 or 120. However, as can be understood from the configuration of encoders shown in FIG. 5, the generation of each of codes 113 and 114 requires two images having different resolution values. Hence, for generating code 110, the generation of codes 113 and 114 must be delayed and the images having various resolutions must be stored in memory while the generation of code 112 is completed. Such processing requires a large-capacity image memory.
On the other hand, when transmitting code 120, a memory is not needed at the encoding side for separately storing an image having the highest resolution while encoding of an image having the lowest resolution is completed, and the memory therefore can be sequentially cleared for storing an image of low resolution. However, since the decoding side must first perform decoding from data for an image of low resolution, it is necessary to provide a memory to temporarily store data received for an image of high resolution, and therefore to perform complicated processing.